1. Field of the Invention
The present invention relates to a thermal assisted magnetic recording head provided with a magnetic head slider and a light source unit, and more particularly to a photodetector included in the light source unit.
2. Description of Related Art
There has been necessary to improve the performance of thin-film magnetic heads in connection with increasing the recording density of hard disk apparatuses. A magnetic recording medium is a discontinuous medium consisting of an aggregate of magnetic fine particles each having a single magnetic domain structure. Each recording bit on the magnetic recording medium is composed of a plurality of magnetic fine particles. In order to increase recording density, asperity of the border between adjacent recording bits needs to be reduced. To realize this reduction, the size of the magnetic fine particles needs to be decreased. On the other hand, the magnetization of small, fine particles, namely, magnetic fine particles small in volume, leads to reduced thermal stability. An effective solution to this problem is to increase the anisotropic energy of the magnetic fine particles. However, higher anisotropic energy of magnetic fine particles enhances coercivity of the magnetic recording medium, making it difficult to record information with the existing magnetic head.
So-called thermal assisted magnetic recording is proposed as a method for solving this problem. This method allows the use of magnetic recording medium having high coercivity. When recording information, magnetic field and heat are simultaneously applied to an area of the magnetic recording medium where information is to be recorded, thereby raising the temperature of the area. In the area where coercivity has thus been decreased, the magnetic field enables information to be recorded. Hereinafter, a magnetic head used for thermal assisted magnetic recording will be called a thermal assisted magnetic recording head.
The thermal assisted magnetic recording head disclosed in U.S. Pat. No. 8,687,469 has a magnetic head slider that records information on a magnetic recording medium and a light source unit that is fixed to the magnetic head slider and that supplies laser light to the magnetic head slider. The light source has an edge emitting-type laser diode mounted on a substrate. The edge emitting-type laser diode emits laser light (forward light) from a surface that faces the magnetic head slider, and at the same time, emits laser light (rearward light) from the back surface. The magnetic head slider has near-field light generation means. The near-field light generation means propagates the forward light that is emitted from the light source unit and generates near-field light on the air bearing surface. The near-field light generation means has a waveguide for propagating the forward light and a plasmon generator for generating the near-field light.
The plasmon generator tends to reach high temperatures, causing not only damage to itself but also damage, cracks, etc. to the dielectric body that surrounds the plasmon generator. It is therefore necessary to properly control the output of the forward light. U.S. Pat. No. 8,687,469 and JP2013-62468A disclose a light source unit that has a photodetector on the back surface side of a laser diode. Since the ratio of light intensity between the forward light and the rearward light is previously known, it is possible to control the light intensity of the forward light by measuring the light intensity of the rearward light.